A conventional two-dimensional electron gas FET is schematically shown in FIG. 1. On a semiconductor substrate 1, an undoped channel layer 2 of, for example, GaAs or InGaAs and an n-type doped electron supply layer 3 of, for example, InAlAs, AlGaAs, or GaAs are stacked. On electron supply layer 3, a source electrode 4, a drain electrode 5, and a gate electrode 6 are disposed.
As known well, electron supply layer 3 is made of a material having a smaller electron affinity than the material of channel layer 2. Thus, in this FET, carriers generated by donors, with which electron supply layer 3 is doped, move into undoped channel layer 2 so that a two-dimensional electron gas layer (2-DEG) 7 which, in view of quantum mechanics, has lost its freedom in the z-direction Is formed in channel layer 2. Because the two-dimensional electron gas (2-DEG) moves in channel layer 2 which has not been doped with an impurity, little impurity dispersion occurs and, therefore, the electron mobility is large. This means that this FET can operate at a high speed.
Although scattering in the z-direction can be suppressed when the electron gas moves from a source region in channel layer 2 beneath source electrode 4 toward a drain region in channel layer 2 beneath drain electrode 5, it is subjected to scattering in the y-direction or scattering including y-direction components as indicated by broken line arrows, in FIG. 1. This poses limitations on increasing the operating speed and on suppressing noise.
As described above, the conventional semiconductor device shown in FIG. 1 has a defect that because the electron gas is subjected to scattering including y-direction components, further improvement of high-speed and low-noise operation cannot be made. An object of the present invention is to provide a semiconductor device in which the degree of freedom not only in the z-direction but also in the y-direction is removed to make the electron gas in a channel layer one-dimensional so that the semiconductor device can operate at a higher speed and with less noise.